TECHNICAL FIELD

The invention relates to drive assemblies for fuel pumps in internal combustion engines. In particular, the invention relates to a drive assembly for a fuel pump including a tappet and a roller. The invention also relates to a tappet for a drive assembly.

BACKGROUND

Tappets are well known in engineering fields for transferring rotating motion into linear motion. In fuel injection pumps, a tappet can be used to transferring the rotating motion of the camshaft to the reciprocating linear motion of the pumping plunger to pressurise fuel to be delivered to a common rail. The camshaft bears an eccentric cam, which is in contact with a roller carried by the tappet. As the camshaft rotates, the eccentricity of the cam causes the tappet to move upwards and downwards as the contact point between the cam and the roller traces a reciprocating vertical path. The tappet is coupled to the plunger so the vertical motion of the tappet is mirrored by the plunger. Tappets are usually received within a shaft of a housing and it is important that the tappet body has a high degree of cylindricity to ensure consistent flow of lubricant around the outside of the tappet so that vertical movement of the tappet within the shaft is efficient.

Tappets can also be used in a similar way to drive the periodic opening and closing of a valve assembly.

It is against this background that the present invention has been devised.

SUMMARY OF THE INVENTION

According to a first aspect, the invention provides a drive assembly for a fuel pump for an internal combustion engine. The drive assembly comprises: a housing and a tappet assembly received within a tappet bore provided in the housing. The tappet assembly comprises: a tappet body comprising a cavity; a roller assembly housed within the cavity, the roller assembly in turn comprising: a pin extending transversely across the cavity, through the tappet body and the roller assembly; and a roller mounted on the pin. At least a first end of the pin extends laterally beyond the outer diameter of the tappet body and is received in a first axial slot within the housing such that rotation of the tappet assembly about a longitudinal axis thereof is prevented.

The first end of the pin forms a close fit within the first axial slot in the housing in order to prevent the rotation of the tappet assembly about its longitudinal axis. For example, the width of the first axial slot may be between 5% and 10% greater than the diameter of the pin at the first end, including that the width of the first axial slot may be, for example, be 10% greater than the diameter of the pin at the first end. However, the difference between the width of the first axial slot and the diameter of the pin at the first end may be smaller than this, for example on 2% or 3% greater than the diameter of the pin at the first end.

The first end of the pin may extend laterally beyond the outer diameter of the tappet body by approximately 10% of the length of the pin.

A second end of the pin may also extend laterally beyond the outer diameter of the tappet body on an opposing side thereof to the first end and may be received in a second axial slot within the tappet bore. As above with the first end of the pin and the first axial slot, the width of the second axial slot may be between 5% and 10% greater than the diameter of the pin at the second end, or only 2% or 3% greater than this diameter.

The second end of the pin may extend laterally beyond the outer diameter of the tappet body by approximately 10% of the length of the pin.

An axial clearance between each of the first and/or second ends of the pin and an axial face of a respective one of the first and/or second axial slots may be no greater than 1 mm. This helps ensure that a sufficient load bearing surface of the ends of the pin within the respective slots is provided.

In order to reduce wear, at least the first end of the pin may comprise a low friction coating, surface treatment or finish, which may be a diamond-like carbon coating.

The drive assembly may further comprise a lubrication slot in the tappet body to help distribute lubricant around the tappet assembly. The lubrication slot may comprise a drilling through the tappet body that opens into the cavity so that lubricant can also be provided to the roller assembly housed therein. The roller assembly may further comprise a bushing mounted around the pin within the cavity between the pin and the roller.

According to a second aspect, the invention provides a method of assembling the drive assembly discussed above on a camshaft. The method comprises: receiving the roller of the roller assembly within the cavity; inserting the pin of the roller assembly through a first opening in the tappet body, through the roller and through a second opening in the tappet body on an opposing side to the first opening to mount the roller on the pin, thereby forming a tappet assembly. The pin is inserted through the tappet body such that at least the first end of the pin extends laterally beyond the tappet body. Subsequently, the method comprises: inserting the tappet assembly into the housing via a low-pressure end of the tappet bore such that the first end of the pin is received in the first axial slot; retaining the tappet assembly within the tappet bore with a temporary retaining means; inserting the camshaft into the housing at the low- pressure end of the tappet bore; releasing the tappet assembly from the temporary retaining means such that the roller rests against the camshaft; and removing the temporary retaining means from the drive assembly.

It will be appreciated that preferred and/or optional features of the first aspect of the invention may be incorporated alone or in appropriate combination within the second aspect of the invention also.

BRIEF DESCRIPTION OF THE DRAWINGS

So that it may be more readily understood, the invention will now be described with reference to the accompanying drawings, in which:

Figure 1 is a perspective view of a tappet assembly forming a part of a drive assembly for a fuel pump in the prior art;

Figure 2 is a perspective view of a tappet assembly forming part of a drive assembly for a fuel pump according to a first embodiment of the invention;

Figure 3 is a cross sectional view of a drive assembly incorporating the tappet assembly of Figure 2; and

Figure 4 is a section view of the pump housing of the drive assembly of Figure 3 to show axial slots formed in the housing. DETAILED DESCRIPTION

Throughout this description, it should be understood that use of relative positional terms, such as ‘upper’, ‘lower’, ‘top’, ‘bottom’, ‘above’, ‘below’, ‘upward’, ‘downward’ and the like are not intended to be limiting and relate only to the orientation of the relevant components as shown in the drawings.

Figure 1 shows a conventional tappet assembly 110 for use in a drive assembly of an internal combustion engine to drive a plunger (not shown in Figure 1) in a fuel pump. In use, the tappet assembly 110 is received within a generally cylindrical tappet shaft (often referred to as the tappet bore) that extends through a housing of the fuel pump. The tappet assembly 110 is mounted on a cam shaft and is free to move vertically within the tappet bore, as dictated by the rotary action of the cam, to drive motion of a pumping plunger of the fuel pump. The tappet bore has a relatively low-pressure end, which communicates with the shaft for receiving the camshaft, and a pump end towards the plunger (not shown).

The tappet assembly 110 comprises a generally cylindrical tappet body 112 defining a tappet body outer diameter (OD), which typically defines the maximum width 114 of the tappet assembly, for reasons that will become apparent in the description below. Towards a lower end of the tappet body 112, a pair of levelled portions 116 are provided on opposing sides of the tappet body 12 (only one of the levelled portions is visible in Figure 1). The levelled portions 116 provide a pair of flat surfaces through which one of a corresponding pair of circular openings 118 extends through the walls of the tappet body 112 (only one of the circular openings 118 is shown in Figure 1). A lower tappet cavity is defined at the lower end of the tappet body 112. The lower tappet cavity houses a roller assembly, referred to generally as 122.

The roller assembly 122 comprises a pin 124, a bushing (not shown) and a roller 128. The pin 124 passes through the lower tappet cavity such that the ends of the pin (only one of which is visible) are received within a respective one of the circular openings 118 in a fixed positional and rotational relationship therewith, as will be further explained below. The ends of the pin 124 extend only to the circular openings 118 and do not extend beyond the tappet body OD 114. The bushing is freely mounted around the pin 124 within the lower tappet cavity, such that the bushing may rotate around the pin 124. The roller 128 is in turn mounted freely around the bushing inside the lower tappet cavity, such that the bushing provides a bearing surface for the roller 128 such that the roller can rotate freely around the bushing. The roller 128 is therefore able to freely rotate around the bushing and the pin 124 to follow the rotary action of a cam mounted on a camshaft (not shown), as is conventional.

The tappet body 112 includes a pair of lubrication slots 130 which act to distribute lubrication oil around the outer diameter of the tappet assembly as it moves along its axis, in use. Only one of the lubrication slots 130 is shown in Figure 1. The lubrication slot 130 communicates with a circumferential groove or recess 131 which extends around the full circumference of the tappet body 12. The lubrication slot 130 and circumferential groove 131 help deliver lubricant to the outside of the tappet body 112, as well as to the roller assembly 122, as will be discussed in further detail with respect to the invention below.

The tappet body 112 also includes an elongate recess or slot 132 which extends axially partway along the length of the tappet body 112 from a lower end thereof. The slot 132 forms a part of an anti-rotation feature for the tappet assembly 110, as will be explained in further detail below.

To assemble the tappet body 112 on the pin 124, the roller 128 and the bushing are held within the lower tappet cavity and the pin 124 is pushed through one of the circular openings 118 in the wall of the tappet body 112, through the bushing and into the circular opening 118 on the opposing side of the wall of the tappet body 112. To retain the pin 124 within the circular openings 118, a caulking operation is carried out, involving the application of a force to deform the ends of the pin 124 such that they are fixed in place in the circular openings 118 and the pin 124 cannot then move axially, nor rotate around its own axis inside the circular openings 118.

The diameter of the tappet bore substantially matches that of the tappet body 112, but is marginally wider than the tappet body 112 to allow for easy vertical movement of the tappet assembly 110. The need for the tappet body 112 to move vertically within the tappet bore constrains the features of the tappet body 112 to be contained within the tappet body OD 114 and not extend beyond it.

The housing also includes a protrusion (not shown) extending into the tappet bore, which is received in the recessed slot 132 in the tappet body 112. Cooperation between the protrusion and the recessed slot 132 serves to prevent rotation of the tappet body 12 about its longitudinal axis as it is driven within the tappet bore. The recessed slot 132 and the protrusion thus act together as an anti-rotation feature and as such may individually be referred to as an “antirotation slot 132” and an “anti-rotation pin” (not shown in Figure 1). Preventing rotation of the tappet body 112 about its longitudinal axis is important to allow optimal engagement of the roller 128 with the cam.

To assemble the tappet body 112 within the housing, the tappet body 112 is inserted into the housing from the upper, high-pressure end of the tappet bore (i.e. the end of the tappet bore located furthest from the camshaft) until the base of the roller 128 engages with the camshaft. The anti-rotation pin engages with the anti-rotation slot 132 to ensure correct rotational placement of the tappet body 112 within the tappet bore, with the cam acting as an abutment for the roller 128 to rest against to ensure correct vertical positioning of the tappet assembly.

As previously mentioned, it is important that the tappet body 112 has a high degree of cylindricity to ensure consistent flow of lubricant around the outside of the tappet body 112 so that vertical movement of the tappet body 112 within the tappet bore is efficient. However, the presence of the anti-rotation slot 132 in the tappet body 112 creates issues for the grinding operation used to create the outer diameter of the tappet body 112 and leads to the creation of a tappet body 112 with poorer cylindricity (i.e., the ideally circular cross-section of the tappet body 112 is more out of round). In addition, the caulking operation used to retain the pin 124 and the bushing within the tappet body 112 also creates appreciable distortion of the outer diameter 114 of the tappet body 112, which also creates a cylindricity penalty.

Figures 2 shows a tappet assembly 10 according to an embodiment of the invention which addresses the aforementioned shortcomings. Figure 3 shows a drive assembly housing a number of tappet assemblies 10 such as the one shown in Figure 2. The right-most tappet assembly 10 in Figure 3 shows the tappet body in a first operating position in the tappet drive cycle as the tappet assembly 10, including a tappet body 12 is driven between bottom dead centre (BDC) and top dead centre (TDC). The tappet drive cycle includes a pumping stroke, in which the tappet assembly 10 and tappet body 12 are driven upwardly within the tappet bore 52 towards TDC, and a return stroke, in which the tappet assembly 10 and the tappet body 12 are urged to move downwardly within the tappet bore 52 towards BDC. Moving from right to left in Figure 3, the tappet assemblies 10 are shown at consecutively later positions in the drive cycle, closer to the BDC position.

As seen in Figure 3, the tappet body 12 is received in a housing 50 of the pump assembly, with the housing 50 itself shown in greater detail in Figure 4. The housing 50 comprises a generally cylindrical shaft, or tappet bore 52, extending therethrough, which receives the tappet body 12 in a similar way to the prior art arrangement described in relation to Figure 1. Vertical movement of the tappet body 12 is driven by means of a rotating cam 54a carried on a cam shaft 54 which extends through a hollow shaft in the housing 50. The cam shaft 54 is oriented laterally to the vertical axis of movement of the tappet body 12.

As the cam shaft 54 rotates about its axis and the roller 28 rides over the surface of the cam 54a, the rotary motion of the cam shaft 54 is translated into vertical motion so that the tappet body 12 moves axially (in the illustration shown) within the tappet bore 52. The axial motion of the tappet body 12 then drives the reciprocating action of a pumping plunger (not shown) of the fuel pump. The tappet bore 52 has what may be considered as a low-pressure end 52a, which communicates with the shaft in which the camshaft 54 is located, and a pump end 52b located towards the plunger of the pump (the plunger is not shown in Figure 3). A return spring 56 (not shown in Figure 3) serves to urge the tappet body 12 towards the BDC position, hence performing a return stroke.

As will be apparent from Figure 2, the tappet assembly 10 shares many features and structural similarities with the tappet assembly 110 of the prior art shown in Figure 1 . Much like the tappet assembly 110 of Figure 1 , the tappet body 12 in Figure 2 defines a generally cylindrical body which itself defines a tappet body outer diameter (OD) 14. The tappet body 12 is provided with levelled portions 16 towards a lower end thereof, on opposing sides of the tappet body 12, and a circular opening 18 extends through a respective one of the levelled portions 16 through the walls of the tappet body 12.

As with the tappet body 12 of Figure 1 , a roller assembly, referred to generally as 22, is mounted on the cam 54a of the cam shaft 54. The roller assembly 22 resides within a lower tappet cavity 60 of the tappet assembly 10.

The roller assembly 22 comprises a pin 24 and a roller 28. Each end of the pin 24 is received through a respective one of the circular openings 18 and passes through the lower tappet cavity 60, with the roller 28 freely mounted around the pin 24 within the lower tappet cavity 60, so that the roller 28 can rotate around the pin 24. Optionally, a bushing 26 may be provided as part of the roller assembly, mounted around the pin 24 between the pin 24 and the roller 22, to provide an additional bearing surface for the roller 22. Unlike the roller assembly of Figure 1 , the first and second ends 24a, 24b of the pin 24 extend beyond the tappet body OD 14 as seen in Figure 3. The pin 24 is also free to rotate within the circular openings 18. The pin 24 is provided with a central drilling 62 which extends from one end of the pin to the other so that the pin 24 forms a tube. The extension of both the first and second ends, 24a, 24b of the pin 24 beyond the tappet body OD 14 shares the rotational load between two faces of the pin 24 and prevents any bias in loading to one side of the pin 24, which may affect the concentricity of the pin 24 with the roller 22 and the bushing 26 if present. Any impact on the concentricity of the pin 24 within the roller 22 can affect the development of the hydrodynamic film between these components from the lubricant present in the system, which may lead to differential wear across the components. Although it is therefore preferable for both the first end 24a and the second end 24b of the pin 24 to extend beyond the tappet body OD 14 in a symmetrical manner, it is possible for only one end 24a of the pin 24 to extend beyond the tappet body OD 14. Subsequent description of the tappet body 12 will consider a scenario where both ends 24a, 24b of the pin 24 extend beyond the tappet body OD 14, although the skilled person will recognise how the same principles may be applied to a tappet body 12 where only one end of the pin 24 extends beyond the tappet body OD 14.

As can be seen in Figure 2, and in a similar way to the tappet assembly 110 of Figure 1 , a longitudinal lubrication slot 30 is provided on the tappet body 12 and communicates with a circumferential groove 31 in order to deliver lubricant to the outside of the tappet body 12 and to the roller assembly 22. The lubricant is delivered to the tappet bore 52 by a radial feed drilling 58 in the housing 50 (visible in Figure 4), which interfaces with a longitudinal slot in the tappet body 12 (not shown). In this way, lubricant is provided between the tappet body 12 and the tappet bore 52. The lubricant is also able to flow from this longitudinal slot into the circumferential groove 31 and to the lubrication slots 30. From there, the lubricant is able to flow into a drilling 32 through the tappet body 12 and into the lower tappet cavity 60 to provide lubrication to the parts of the roller assembly 22.

The tappet bore 52 additionally includes a pair of opposed axial slots 44a, 44b, recessed into the bore 52, that extend into the tappet bore 52 from the low-pressure end 52a. When the tappet body 12 is received in the tappet bore 52, each end 24a, 24b of the pin 24 which extends beyond the tappet body OD 14 is received within a respective one of the slots 44a, 44b. Each slot 44a, 44b defines an axial face, in parallel with the axis of the tappet body 12.

The slots 44a, 44b extend lengthways into the tappet bore 52 only as far as is necessary for the full range of vertical motion of the tappet body 12 to be realised. The width of the slots 44a, 44b, in a direction perpendicular to the lengthwise direction of the slots 44a, 44b, must be similar to the diameter of the pin 24 such that the ends 24a, 24b of the pin are received within the respective slot 44a, 44b in a close fit therewith in order to prevent rotation of the tappet assembly 10 within the tappet bore 52. For example, the width of the slots 44a, 44b may be up to and including approximately 10% greater than the diameter of the pin 24 at the ends 24a, 24b. However, the width of the slots 44a, 44b may in other embodiments be less than 10% greater than the diameter of the pin 24. For example, the width of the slots 44a, 44b may be between 5% and 10% greater than the diameter of the pin 24, or may be a lower value, such as 2% or 3% greater than the diameter of the pin 24.

With regard to the length of the pin 24, approximately 10% of the length of the pin 24 protrudes from the tappet body 12 at each end 24a, 24b into the respective slots 44a, 44b. The axial clearance between the axial faces of the slots 44a, 44b and the ends 24a, 24b of the pin 24 is minimised in order to present a maximum load-bearing surface on the pin ends 24a, 24b in the slots 44a, 44b for anti-rotation functionality and so that the pin 24 is not able to slip out of the circular openings 18 of the tappet assembly 10. Preferably, the axial clearance between the length of the pin 24 and the distance between the axial faces of the slots 44a, 44b is not greater than 1 mm.

The close fit between the ends 24a, 24b of the pin 24 and the slots 44a, 44b prevents rotation of the tappet body 12 about its longitudinal axis. Accordingly, the slots 44a, 44b and the pin 24 together act as an anti-rotation feature and the slots 44a, 44b and the pin 24 may be referred to as “anti-rotation slots” and an “anti-rotation pin”, respectively. As the tappet assembly 10 moves through the drive stroke, between BDC and TDC, under the influence of the rotating camshaft 54, and back again through the return stroke between TDC and BDC, under the influence of the return spring, the ends 24a, 24b of the pin 24 therefore travel through the respective axial slots 44a, 44b, preventing an angular movement of the tappet body 12 about its axis. At the top of the drive stroke (i.e. at TDC), the pin ends 24a, 24b are at the top of their respective axial slot 44a, 44b.

The ends 24a, 24b of the pin 24 may be coated in a low friction coating in order to reduce wear between the ends 24a, 24b of the pin 24 and the slots 44a, 44b. Any wear between the ends 24a, 24b of the pin 24 and the slots 44a, 44b may loosen the close fit therebetween and enable rotation of the tappet body 12 about its longitudinal axis. A suitable low friction coating may be diamond-like carbon (DLC).

To assemble the tappet assembly 10, the roller 22 is held within the lower tappet cavity 60, with the pin 24 pushed through one of the circular openings 18 in the tappet body 12, through the bore which extends through the roller 28 and through the other, opposing circular opening 18. No caulking step is applied to the ends of the pin, enabling the pin 24 to rotate around its own axis. From this point, the drive assembly is assembled by inserting the tappet body 12 into the tappet bore 52. However, the extension of the ends 24a, 24b of the pin 24 beyond the tappet body OD 14, and the limited vertical extent of the slots 44a, 44b in the housing 50, prevents the tappet body 12 from being inserted into the tappet bore 52 from the plunger end 52b of the tappet bore 52. Instead, before the camshaft is incorporated into the drive assembly, the tappet body 12 must be inserted into the tappet bore 52 from the low-pressure end of the bore 52, with the ends 24a, 24b of the pin 24 being received in the slots 44a, 44b, respectively.

During this phase of operation, as there is no camshaft 54 for the roller 28 to rest against, the tappet body 12 must be retained within the tappet bore 52 by alternative means until the camshaft 54 is assembled into place. To this end, the tappet body 12 may be temporarily held in position by temporary retaining means (not shown) to ensure it is held in place before assembly onto the camshaft 54. For example, the tappet assembly 10 may be drawn up the tappet bore 52 by a cradle and then lightly held by a clamp while the cradle is removed, and the camshaft 54 is inserted through the hollow. The tappet body 12 may then be released from the clamp so that the roller 28 can rest against the camshaft 54. In this scenario, therefore, the clamp acts as the temporary retaining means, although the skilled person will realise that this function may be provided by an alternative component.

The tappet assembly 10 of the invention therefore represents a shift in engineering philosophy compared to the prior art in relation to the provision of anti-rotation features. Typically, antirotation features have been provided as a male feature, such as a pin, on the housing 50, extending into the tappet bore 52 to engage a corresponding female feature, such as a slot, on the tappet body 12. The anti-rotation feature of the invention therefore exists in contrast to this orthodoxy, providing male features (e.g. extended pin 24) as part of the tappet assembly 10, with corresponding female features in the housing 50.

In particular, by providing the ends 24a, 24b of the pin 24 as the male part of the anti-rotation feature, the number of processing steps applied to the outer surface of the tappet body 12 after it is initially formed is reduced and the caulking step applied to the ends 24a, 24b of the pin 24 is removed. Both of these factors contribute to improve the cylindricity of the tappet body 12 and enable more consistent flow of lubricant around the outer surface of the tappet body 12.

The removal of the caulking step also greatly simplifies the assembly of the tappet assembly 10 and avoids issues surrounding the difficulty of controlling the force applied to the ends of the pin 24 during the caulking step. By removing the caulking step, the pin 24 is enabled to rotate around its own axis within the circular openings 18; this increases the durability of the tappet body 12 by allowing the bending load to act on different parts of the pin 24, in contrast to a caulked pin that is fixed relative to the tappet body. It also enables a worn pin to be replaced as the pin can simply be removed from the tappet assembly 10.

It will be appreciated that various alternative embodiments of the invention may be envisaged without departing from the scope of the appended claims.